Rotating shaft devices are used extensively throughout the world, and have been improved many times over the course of many years. However, as described below with respect to centrifugal pumps, rotating shaft devices still have considerable problems which have not been resolved.
Centrifugal pumps typically contain an impeller coupled to a rotating shaft. In conventional centrifugal pumps a pumpable fluid (pumpate) is pulled into the eye of the impeller where the fluid is spun radially outwards into the volute space. Pressure builds in the volute space until the pressure is able to overcome the discharge resistance, at which point the pumpate exits the pump.
It is well known in the art to use either a soild or a hollow shaft, and to rotate the shaft using electromotive forces. Typically, a rotor is affixed to the outer circumference of the shaft of either configuration for this purpose, and the rotor is acted upon by a concentric stator. Either arrangement raises the possibility that some of the pumped fluid will leak from spaces between various stationary and rotating components.
It is known to eliminate the leakage problem by utilizing a canned pump (not shown). In such pumps the rotor and shaft bearings are contained within a "can" inside the pumpate flow stream. Canned pumps are typified by U.S. Pat. No. 3,667,870 to Yoshida, Canadian Pat. no. 733,312 to Penman, and more recently U.S. Pat. No. 5,356,273 to Nixon. While canned pump designs are effective in addressing the leakage problem, they are inherently inefficient because the wall of the "can" is interposed between the rotor and the stator. Such a design necessarily reduces the efficiency of electromagnetic energy transmission.
In centrifugal pumps other than canned pumps, leakage is commonly addressed using mechanical seals. Prior art FIG. 1 depicts a generic centrifugal pump 110 having a rotating shaft 120 to which is affixed an impeller 130. A rotor 140 is concentric about the rotating shaft 120, and a motor stator 150 is concentric about rotor 140. A volute 160 is held away from the motor assembly by close couple frame 111. Volute 160 partially encloses the impeller 130, and has suction inlet 15 and discharge outlet 165. Fluid enters the pump in the direction of arrow 105. Centrifugal pump 110 has a generic mechanical seal 110A which includes non-rotating secondary seals 170 and 171, and rotating secondary seals 172 and 173. In the relationship shown, non-rotating secondary seal 170 seals non-rotating seal 171, and rotating seal 172 seals rotating secondary seal 173. A rotating pressure spring 174 biases seal 171 against seal 172, and a spring cup 175 helps stabilize the pressure spring 174.
For centrifugal pumps of the type shown in FIG. 1, there are logically only two categories of points A and B (shown) at which the seals can be positioned with respect to the rotating shaft 120 entering the volute to prevent leakage. In either position mechanical seals are positioned to provide a seal between the pressurized volute and the rotating shaft. Examples of such pumps are found U.S. Pat. No. 5,288,215 to Chancellor et al. (the '215 patent). In the '215 designs all of the fluid within the pressurized volute is prevented from recirculating back into the impeller suction eye. This provides a high degree of operational efficiency relative to pumps that allow pumpate recirculation lose optimum design efficiency through the pumpate recirculation.
Mechanical seals in centrifugal pumps are plagued with design problems because such seals necessarily involve at least two lap seal finished faces rubbing against each other. In FIG. 1, for example, seal 172 rubs against seal 171, and such surfaces tend to wear out. This problem is especially acute where the pumpate includes suspended solids. Seal service life in conventional mechanical seals can also be reduced because of misalignment of the seal faces due to shaft deflection, and seal run out. One partial solution involves the placement of a "floating" or "intermediate" double faced seal between a rotating sealing surface and a stationary sealing surface. This can reduce the relative sliding speed between adjacent sealing surfaces to about one-half the speed encountered with a simple two-surface seal. Exemplary disclosures in this area are U.S. Pat. No. 4,351,533 to Moore and U.S. Pat. No. 4,266,786 to Weise. It is also known for seal designers to utilize a self-lubricating sealing material, which requires no lubricant. Such materials can be used as combination bearing-seals, and an example of such materials are described in U.S. Pat. No. 4,764,035 to Boyd.
Biasing of the seals against one another produces yet additional problems. Biasing is typically accomplished by spring loading one of the sealing surfaces, but may also be accomplished using another force such as fluid pressure as described in U.S. Pat. No. 4,707,150 to Graham Biasing of one sealing member against another is generally made axially but not radially.
It is known to incorporate several of the above-described improvements into a centrifugal pump having the first category of seals described above. The above-mentioned '215 patent, for example, describes a pump having an advanced axially biased floating seal. Despite all of these improvements, however, there is still a need to provide improved seals in a centrifugal pump. The above-described problems are largely echoed in other rotating shaft devices. Commonly used axles, for example, are typically supported by one or more lubricated bearings, which have sealing problems with respect to the lubricant.